1. Field of the Invention
The present invention relates, in general, to integrated circuits and, more particularly, to an integrated circuit having indirect sensing of the average current in a synchronous-buck power stage.
2. Relevant Background
Switching regulators, including ripple regulators, are commonly used because of their characteristic high efficiency and high power density (i.e., power-to-volume ratio) resulting from smaller magnetic, capacitive, and heat sink components. Switching regulators indirectly regulate an average DC output voltage by selectively storing energy by switching energy on and off in an inductor. By comparing the output voltage to a reference voltage the inductor current is controlled to provide the desired output voltage.
Synchronous buck power stages are a specific type of switching regulator that use two power switches such as power MOSFET transistors to control current in the output inductor. A high-side switch selectively couples the inductor to a positive power supply while a low-side switch selectively couples the inductor to ground. A pulse width modulation (PWM) control circuit is used to control the high-side and low-side switches. Synchronous buck regulators offer high efficiency when low on-resistance power MOSFET devices are used.
In many applications it is desirable to measure the amount of current sent to a load by the regulator. This information can be used to protect the load and/or regulator circuit from damage caused by excessive current. This information can also be used to monitor the state of the load. For example, load current indicates power used by a motor and can indicate if a motor is overloaded or otherwise faulty. Load current also may indicate a circuit board with faulty connections or components.
One prior solution uses low value sense resistors placed in series with the regulator output. The load current can be monitored by sensing the voltage drop across the sense resistor. This solution increases cost of the circuit. Further, the sense resistor reduces power efficiency that both wastes energy and creates heat that must be removed from the system. Another solution uses current sensing power field effect transistors in the output stage switches. While this solution provides better power efficiency than sense resistors, it often is not cost effective. A need exists for a cost effective structure and method for sensing current in a regulator that does not require the use of sense resistors.